The purpose of this section is to provide design guidance for designing high performance assemblies. This section relies on many other industry standards, which should also be consulted. It is the intent to provide recommendations which go beyond the content of those standards, especially as it relates to integrating assemblies into a total building enclosure design. It is intended to provide a "Best Practice" and shall not be construed in any manner to establish the legal standard of care required from licensed professionals.

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Introduction

A wide variety of panelized metal wall systems are available for installation as a buildings exterior wall cladding. Each system must be specially adapted to its intended building use. Metal wall panels are usually fabricated of aluminum but can also be manufactured from steel, stainless steel, copper, or composite materials.

The following types of metal panel systems are available:

Lap-Seam Metal Panels

These panels are formed out of metal sheets and are typically ship-lapped with adjacent panels. At panel edges, gutters or sealant tape are typically included as part of the system. The panels are typically erected in strips up to 4 feet wide and 20 feet long. The thickness of the metal sheet is generally less than 0.05 inches (1.10 mm.)

Composite Metal Wall Panels

These panels feature two sheets of metal adhered to a core material. These composite panels are usually stronger than lap-seam panels and provide built-in insulation to the wall system. The thickness of the metal sheet is generally less than 0.05 inches (1.10 mm.) The overall panel thickness varies, depending on the insulation, from 1.2 inch to over 2 inches. Composite metal wall panels are often ship-lapped with adjacent panels similar to lap-seam panels. Types of composite panels include:

Foamed-Insulation Core Metal Wall Panels: These panels have metal facings with insulation foamed into place during fabrication.

Laminated-Insulation Core Metal Wall Panels: These panels have the metal facings laminated to preformed insulation boards. Panels can have tongue-in-grove joints for interface with adjacent panels or framed edges where perimeter extrusions attach the panels.

Flat Plate Metal Wall Panels

These panels are typically fabricated out of 1/8 inch thick metal plate. The panels are then bent to the desired profile. Stiffeners and support structure can be welded or adhered to the flat plate. Since these panels are manufactured out of plates, they have very high impact resistance and durability compared to other types of panels. Some manufactures will adhere flat plate to metal extrusions to create their system.

Metal-Faced Composite Panels

These panels consist of metal facings adhered to a thin thermoplastic core. The thickness of the metal sheets is typically less than 0.05 inches (1.10 mm.) and the overall panel typically is up to 1/4 inches thick (6 mm). The resulting composite panel is then bent to the desired profile. These panels are not as impact resistant as flat plate; however, depending on the size of the panels, stiffeners can be welded or adhered to the rear surface of the panels.

The size of flat plate and metal-faced composite panels is generally less than 10 feet by 10 feet. These panels are generally fastened by proprietary installation systems.

The thickness of the system depends on the structural support system for the panels. The greater the size and span of the panel, the deeper the thickness of the system. The thickness of the metal panel system can range from 2 inches deep for small panels to over 6 inches deep if the panels are large and need to span end to end between supports. The panels can be either directly fastened to the structural system or be fastened to a secondary structural system of metal studs, hat tracks and supporting channels. The hat channel and support system can also be part of the water resistive design for the panels, especially for more complex systems which include flat plate and metal-faced composite panels.

A wide variety of designs are used to prevent water leakage for metal panel systems including face sealed barrier systems, weeped drainage systems, and rainscreens. Rainscreen metal panel designs can be pressure equalized and back-ventilated. In ventilated rainscreens, the majority of water is collected and channeled to the building exterior at the face of the panels, but a back-up membrane must be present to divert water to the exterior.

Description

Metal Types

Metal can be aluminum, stainless steel, copper or steel. Aluminum is the most common material used due to its cost, corrosion resistance, and durability. In high-end applications, stainless steel and copper can also be used. Several manufacturers fabricate steel panel systems, which require protective coatings for resistance to corrosion.

Support and Anchorage Systems

Metal panel systems are engineered to support gravity, seismic and wind loading. The fastening of the panels also needs to accommodate interstory drift requirements in seismic zones. The support system of the panels needs to be able to accommodate tolerance from existing construction and fabrication.

The metal panels are typically screwed or bolted on a structural frame which often consists of metal studs.

Joints and Joint Detailing

Since metal is impervious to water, panel joint design is critical to the water tightness of the system. A metal panel building typically has an extensive number of joints. How the joints perform is a factor of the panel design and construction. If the metal panel system is designed based on a barrier system design, the joints between the metal panels are typically face sealed. If the system is a rainscreen or drainage design, the joints between the panels are typically left unsealed. Some designers select a rainscreen or drainage system for both performance characteristics and the aesthetic criteria of unsealed joints.

Compared to concrete or masonry cladding elements, metal panel systems have higher coefficients of expansion for thermal movement. Designers of metal panel systems need to calculate the expected movement of metal panels due to changes of temperature. For example, a 20 foot long aluminum extrusion may expand or contract 0.30 inches when subjected to a 100 degree temperature change.

Joints between panels must be wide enough to accommodate thermal expansion and differential movements between panels. Joint sizes can vary from 1/4 inch wide for small panels to 1 inch wide for larger panels. Factors that influence joint size include panel size, panel location on the building, and tolerance issues. Generally, larger panels require larger joints than smaller panels.

If the joint needs to be sealed, the metal panel edges must be configured with a return so that a properly designed sealant joint with backing materials can be installed. Sealant does not provide durable performance when adhered to metal panels less than 1/4 inch thick. Sealant joint design must also include provisions for two sided adhesion for joint expansion and contraction.

Common Backup Wall Elements

The following elements are often found in a metal panel cladding system:

Insulation: With the exception of insulated metal panels, metal panels do not typically provide any insulation value to wall systems. When non-insulated metal panels are used, the insulation is typically provided by batt insulation set within the stud wall behind the metal panel cladding.

Air and Moisture Barrier: While face sealed systems do not require an air and moisture barrier, drainage and rainscreen metal panel typically require a secondary air and moisture barrier system. The selection of the air and moisture barrier is an important decision that needs to be customized to the specific nature of the metal panel system, the exterior sheathing, geometry, and HVAC requirements of the building. The air and moisture barrier may include building papers, house wraps, elastomeric coatings, or peel-and-stick waterproofing membranes.

Metal Stud Framing: The design of the metal stud framing needs to be integrated with the panel design and fastening system.

Fundamentals

Metal Panel Design

Metal panel systems are generally proprietary designs in which a manufacturer adapts his system to the architect's design. As part of the design, the architect will select a type of metal panel system and provide architectural details that depict the relationship of the metal panel system to the adjacent building systems. Generally, architects do not provide comprehensive detailing for the metal panel system as part of the construction documents. The project specifications and shop drawings, as well as shop drawing and submittal review, are critical to the success of the project.

When the designers select the metal panel system, they need to determine if the system requires an air and water barrier or rain screen. The air and water barrier requirements need to be clearly shown on the contract documents since the metal panel manufacturer does not provide for these systems or their detailing.

In the specification, the designer must select the types of metal panels that will meet design criteria and also establish the panel performance criteria. Performance criteria need to include:

Wind loading

Seismic design criteria

Deflection criteria

Air infiltration criteria

Water test performance criteria

Panel flatness criteria

Panel tolerance criteria

Thermal movement criteria

Performance testing criteria

Fire resistance ratings, if required

Sound transmission criteria

Insulation criteria

Performance criteria for air and moisture barrier or rainscreen

After the project is bid, the contractor selects a metal panel system based on the specification. The shop drawing review process is critical to the success of a project. During this process, the manufacturer adapts his system to the specific building. The shop drawings are accompanied by structural calculations that need to be performed by a structural engineer licensed in the project state; (this is especially important for states in high seismic zones). During the shop drawing review process, the manufacturer, installer, general contractor, and architect need to review the interface of the metal panels to adjacent systems such as windows and other cladding systems. If the building features a secondary air and moisture barrier or rainscreen, the structural attachment of the metal panels needs to be reviewed to verify that it does not compromise the secondary barrier. During the shop drawing review process, construction tolerances for joint sizes need to be reviewed and accepted by the manufacturer, installer, general contractor and architect.

Structural Aspects of Design

Metal panel systems are typically considered nonstructural elements that are used to form a curtain wall; however, these nonstructural elements typically must be structurally designed.

The panels must be designed to resist out-of-plane loads (such as wind or seismic loads), and must be also designed to resist in-plane loads (seismic loads and vertical/dead loads). Loads from the panels must be transmitted to the building's structural frame. In addition, the panels must allow thermal movements and must be designed so that they do not act to restrain the building's structural system as it deforms under lateral and gravity loads.

Often, metal panel systems are proprietary and the design of the panels is based in part on in-house research by the metal panel manufacturer. Steel panels may be designed using AISC's Specification for the Design of Cold-Formed Steel Structural Members; and aluminum panels may be designed according to The Aluminum Association's Specifications for Aluminum Structures. The design of composite panels may not comply with any standard uniformly recognized by the model building codes and therefore would need to be based on proprietary testing.

Performance Issues

Thermal Performance: Non-insulated metal panel wall systems derive their thermal performance characteristics from the amount of insulation placed in the cavity or within the backup wall.

Moisture protection: Metal panel systems can be barrier systems, drainage systems or rainscreen systems. As discussed in the Joints and Joint Treatments section, the design of metal panel joints is critical for the watertight performance of the panel system.

Fire safety: Metal panel systems are typically not a fire rated system. The design of the metal panel wall systems may need to include fire-stopping at floor levels. Metal panel systems are typically not flammable. Insulated and composite panel systems need to be reviewed to verify that the insulation and thermoplastic cores are not flammable.

Acoustics: Metal panel systems, except for some types of insulated panels, typically do not offer much sound insulation. The design of the back-up wall cavity for metal panels typically provides for sound insulation.

Impact Resistance: With the exception of flat plate metal systems, metal wall panel systems typically have low impact resistance. Metal panels can be damaged or dented by impact loading, which can include swing stage scaffolding. High-rise buildings, which require window washing or periodic maintenance, should be designed with impact resistant panels.

Material/Finish Durability: The finish of the metal panel system is important for system performance. The most common durable finishes applied on metal panels include flouropolymer, powder, and anodized coatings. Flouropolymer and powder coats provide a very wide range of colors as well as color uniformity. Anodizing is generally limited to dark metallic colors for many applicators, and for a large job the color of the panels can vary slightly due to the anodizing process. Generally, shop applied coatings perform much better than field applied coatings.

Another durability consideration is the panel material. Stainless steel, copper, and aluminum panels are constructed of materials that have a high corrosion resistance. Some systems use mild steel panels. If the coating on a mild steel panel fails, the panel will start to corrode. Coating performance is, therefore, critical for steel panel durability. Compared to aluminum and stainless steel, mild steel panels have less durability. Porcelain enamel coatings are often used on steel panels and provide good durability, but these coatings can be vulnerable to damage in the field.

Potential Problems

Pitting: Over time, as metal panels are exposed to weather and pollution, their protective coating can be attacked resulting in a pitted appearance. While the pitting is not a structural concern, the pitting detracts from the appearance of the panel and the building.

Oil Canning: Oil canning is characterized by pillowing or waviness of the metal panel. The oil canning can be caused by problems in fabrication, design, or installation. Oil canning detracts from the appearance of the panel, since part of the selection criteria for metal panels is often flatness. Some glossy panel finishes can also make oil canning more apparent. No industry standards exist to define what degree of oil canning is acceptable. When selecting metal panels, the designer should discuss flatness criteria with the panel manufacturer to order to set design criteria. The panel support design, shipping, and erection process all need to be performed with the intent of limiting oil canning to acceptable levels.

Flat plate metal wall panels are least vulnerable to oil canning compared to other types of panels, which are fabricated out of thin sheets of metal.

Shadowing: Installing welds or stiffeners on the backsides of metal panels can result in shadowing, a condition in which the weld or stiffener is visible on the panel face.

Dissimilar Metals: The use of dissimilar metals can result in two types of problems: water runoff staining and galvanic corrosion.

When water runs off one type of metal onto another, it can stain and corrode the other metal. One example of this is water runoff from copper staining aluminum. Runoff from metal surfaces can also stain some types of stones and other materials.

Galvanic corrosion occurs when one type of metal is in physical contact with another type of metal. The less noble metal will corrode, and this corrosion can affect the panel structural strength. When dissimilar metals are in close proximity, they should either be physically separated or reviewed for galvanic action potential.

Maintainability

Metal panel systems, when properly designed and constructed require little maintenance. However, over the life of the structure need cleaning and sealant replacement are required.

If the system includes sealant, the time frame for sealant replacement usually ranges from 7 to over 20 year periods, depending on the sealant used and the joint design.

Over time, dirt and pollutants will be deposited on metal panels. If the dirt is not removed periodically, pollutants may attack the panel coating system and cause pitting. While pitting on aluminum and stainless steel is not a structural concern, it is unsightly. To mitigate the build-up of pollutants on metal panels, periodic cleaning at approximately 10 year intervals (depending on exposure to dirt and pollutants) is recommended.

Applications

Metal panel wall systems have a wide range of applications and can be designed to achieve a wide range of architectural styles. Their use is appropriate for construction in all climates and environments.

See Appendices for climate-specific guidance regarding building enclosure design.